Plug valves, also known as cocks, are perhaps the oldest type of valve in use today. They are primarily used for quick shutoff of liquid flow. A plug valve is a type of rotary valve that consists of a tapered or cylindrical plug with a quarter-turn rotary motion for starting or stopping the flow.
Plug valves work similarly to ball valves but have a more compact design. They are primarily used in oil, gas, chemical, and water service. While efficient in stopping or starting the flow of liquids, they are not accurate in regulating it. There are several types and designs of plug valves, including lubricated, non-lubricated, eccentric, and expanding plugs. Lubricated plug valves are the most common in industries that work with hard-to-handle liquids due to their tight shutoff.
What is a lubricated plug valve?
Lubricated plug valves use a lubricant (base oil and viscosity improver), injected into the valve under pressure to reduce friction and to provide a seal between the valve’s seating surfaces. Lubricated plug valves are high maintenance as they need to be lubricated regularly. They are therefore recommended for applications with infrequent operations. While the plug is mostly cylindrical, they are also available in conical shapes.
Unlike ball valves or non-lubricated plug valves, Lubricated plug valves are designed with grooves in the plug that retain a lubricant. When in use, the lubricant prevents sticking and provides a hydraulic force that helps lift the plug and reduce the effort required for rotary operation. Additionally, the lubricant provides the seal between the seating surfaces of the valve body and plug so that a tight shutoff can be achieved.
Lubricated plug valve and its working principles
Lubricated plug valves feature a cavity that runs through the centre of the plug and down its axis. The cavity is sealed at the bottom with a sealant injection fitting installed at the helm. These valves use sealants or lubricants to ease their operation in various operations. The lubricant or sealant is injected into the cavity with a check valve usually fitted just below the injection fitting to stop the lubricant from flowing backward.
The valve’s lubricant flows through the radial openings and into grooves on the surface of the plug’s seat. The type of sealant used is determined by the nature of the fluid medium flowing through the pipeline. The correct sealant must be selected and used for the flow medium and operating conditions. When selecting the lubricant, you should exercise caution as the flow medium could easily dissolve or degrade it.
Selecting the wrong lubricant could contaminate the flow medium or cause structural damage to the seal and cause leakage. Moreover, the sealant should not change the medium’s properties or be denatured by the temperature of the flow medium.
Installation guidelines for lubricated plug valves
Before installing your lubricated plug valve, make sure that the characteristics of the valve match those agreed in the specifications. Also, ensure that you properly check the valve for identification purposes. This information is found in the nameplate instructions and name tag stickers on the product casing. If any information is unavailable, immediately contact the manufacturer or the seller.
- You should not remove all wrapping until you are ready to install the valve. If the valve’s protective end caps have been removed for inspection, replace them as soon as the examination is complete. Make sure they are stored far from dust and that they are cleaned and tested before installation begins.
- Remove the valve from its storage and inspect it for any damage that might have occurred during transportation or storage.
- Remove the end protectors of the valve before installing it and check whether the serrations on its flange face are clean and undamaged. If necessary, clean the valve of any debris and scaling. If left in the valve, these elements can damage the valve’s seal and cause leakages when in use.
- Depending on your pipeline’s routing, lubricated plug valves can be mounted horizontally or vertically. Nevertheless, installing the valve with its operator underneath is not recommended because its design does not allow for water and sediment runoff.
- Ensure your pipeline is firmly supported to reduce vibrations and stress in the valve. Ensure that the vibrations do not stress the valve even during tightening. Excessive tightening can easily cause damage to the valve’s end flanges.
Difference between lubricated and non-lubricated plug valves
The main difference between lubricated and non-lubricated plug valves is that the former uses a lubricating system to continually restore the valve seat and extend the life span of the valve seats. Non-lubricated plug valves feature non-metallic thermoplastic or elastomeric liners to reduce friction between the valve’s body and the plug.
Both lubricated and non-lubricated plug valves can provide a bubble-tight liquid shutoff. While they are less efficient in stopping or starting the flow of the medium, non-lubricated plug valves require less maintenance. However, you can’t use them in high-temperature applications due to their non-metallic seats.
Choosing the best valve
Do you need help choosing the best plug valves for your system? Installing or replacing a plug valve in your system is critical to the efficiency of your processes. Before moving forward, you need to know the different types and styles of plugs you need. If you want to install a lubricated plug valve in your system, it is essential to strictly follow the installation guideline and observe all the cautions as indicated in the manual.
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